Frequency modulation receiver



' June 13, 1944. M. G. CROSBY 2,351,192

FREQUENCY MODULATION RECEIVER Filed Max ch 15, 1942 2 Sheets-Sheet l 41FHIPL/FYEE 0e COEEEWON NEW OSCILLfiTOE INVENTOR fiueear' 6 diam-erATTORNEY June 13, 1944. M. e. CROSBY FREQUENCY MODULATION RECEIVER 2Sheets-Sheet 2 Filed March 13, 1942 W H w M w/Z Patented June 13, 19442,351,192 FREQUENCY MODULATION RECEIVER Murray G. Crosby, Riverhead,-N.Y., assignor to Radio Corporation of America, a corporation of DelawareApplication March 13, 1942, Serial No. 434,509

Claims.

This application concerns a frequency modulation receiver in which theincoming signal is heterodyned to another frequency and passed through aretard circuit to superimpose a variable phase shift with frequency upontheheterodyned wave. This wave is heterodyned again with the originalsignal in a manner that removes the frequency variations and leaves thephase variations that were superimposed. These phase variations are thendetected to receive the original frequency modulation signal.

Another method of using this principle is also described which, insteadof making use of the variable-phase-shift-with-frequency principle ofCrosby U. S. Patent 2,229,640, dated January 28, 1941, makes use of theprinciples described in my U. S. Patent 2,060,611, dated November 10,1936. In this latter system a carrier-rejecting circuit is substitutedfor the retard circuit.

In describing my invention more in detail reference will be made to theattached drawings wherein:

Fig. 1 illustrates the essential components of a frequency modulationreceiver arranged in accordance with my invention;

Fig. 2a shows the characteristics of the phase retarding circuit l 6 ofFig. 1;

Fig. 21) illustrates a modified form of circuit for deriving from thefrequency deviations on the wave energy corresponding phase deviationson the wave energy;

Fig. illustrates the characteristics of the circuit of Fig. 2b.

Fig. 1 shows a specific embodiment of the receiving system. Intermediatefrequency output from a superheterodyne receiver I is applied throughtuned transformer 2 to the grids I0 and I2 of respective converter tubesl3 and M. The grid H of converter tube I4 is fed by local oscillator ISwith the result that in mixer tube I 4 the incoming signal isheterodyned down to an intermediate frequency. Transformer [6, whoseprimary circuit is coupled to anode ll of tube M, is tuned to thisintermediate frequency, and acts as a retard circuit to apply a variablephase shift with frequency to the frequency modulated intermediatefrequency wave. This transformer l6 has a phase and amplitudecharacteristic which is shown in Fig. 2a. It is adjusted and damped byresistances 2| and 23 to produce a band-pass action, this beingindicated by curve A of Fig. 2a. The phase characteristic for thisadjustment is such that for the carrier in the mid-frequency position,the output phase is 90 degrees. As the frequency is modulated to eitherside of the carthe incoming signal of intermediate frequency fed to gridID with the phase-frequency modulated wave fed to grid 26. The output ofconverter tube I4 is the difference between a constant frequency wave(supplied by oscillator 15) and the frequency modulated wave supplied togrid l2of tube I so that the output at transformer I8 is frequencymodulated to the same extent to which the wave supplied to grid I2 ismodulated. However, the output of converter tube I3 is the differencebetween the frequency modulated incoming wave on grid In and thefrequency modulated output from converter tube M on grid 26 so that thedifference is constant in frequency. This choice of heterodynefrequencies causes the resulting wave in the output of converter tube [3to have the same frequency as oscillator 15. The output frequency fromtube l3 has the phase modulation component imparted by the retardcircuit but does not have the frequency variations of the originalfrequency modulated signal. This output is fed from plate 30 to tunedcircuit 32, coupled to tuned circuit 34. The phase variations may thenbe detected by means of the phase detector consisting of transformer 3I, choke 36, and diodes 38 and 39. The phase modulated energy from tubeI3 is fed to the push-pull secondary of the input transformer 3| and theunmodulated carrier from oscillator I5 is fed by lead 4| to the commonleg of this input circuit. These two waves are adjusted to be 90 degreesapart for the unmodulated condition with the carrier tuned to the middleof the retard circuit l 8. Since transformer l6 imparts a 90 degreeshift and transformer 3| imparts another 90 degree shift, further phaseadjustment is necessary to produce the proper 90 degree relation betweenthe phase modulated energy fed to tuned circuit 34 and the strippedcarrier or oscillations fed in phase by lead 4| to the diodes 3B and 39.This may be accomplished by means of the adjustment of the size ofcoupling condenser 50 and resistor 45, or by variation of inductance 38and coupling condenser 53, or by a combination of both. If desired, afurther phase adjusting means may be inserted in either the lead inwhich is inserted or the lead in which 53 is inserted.

The principle of operation of the type of phase detection which is usedhere is more completely described in my U. S. Patent 2,081,577, datedMay 25, 1937. Any other type of phase modulation receiver may be usedsuch as that of my U. S. Patent 2,114,335, dated April 19, 1938, or Imay use the method described in my U. S. Patent 2,229,640 in which afrequency modulation receiver is converted into a phase modulationreceiver by means of an audio correction network at the receiver outputterminals. With these last mentioned receivers, the presence of theunmodulated carrier is not necessary so that 'the phase modulationdetector is connected directly to the output of the converter stage,including tube I3, and the lead 4| is omitted. The output of diode sideof Fe.

rectifiers 38 and 39 is now corrected as well as amplified ina stage orstages 43.

Since my receiver may be usedtfor detecting phase or frequency modulatedwaves and waveshaving the characteristics of both, the words phase andfrequency modulation as used herein are to mean the various types ofmodulation wherein the instantaneous frequency of wave energy is variedas a function of modulating potentials. The generic expression anglemodulated. is employed to include these two types of modulation. H

As an example of the operation of the circuit of Fig-1, let it beassumed that the intermediate frequency fed to transformer 2 is 5megacycles and that the frequency of oscillator I5 is 2 mega cycles. Theinterinediate frequency appearing in retard circuit IE will then be 3megacycles and will be frequency modulated'the same number of cycles as.the original incoming signal. This 3 megacycle wave will have thephasevariations imparted to it by retard circuit l6 so that the phase ofthe output will be proportional to the wave frequency at the input. The3 megacycle wave is then heterodyned in tube l3'with the original 5megacycle intermediate frequency to produce a 2 megacycle output whichhas the phase variations imparted by the retard circuit, but thefrequency variations have been removed since the heterodyning frequencyused in converter [3 had the same frequency variations as theoutput of l5 so that the difference frequency is constant. It will be' apparentupon investigation that converter tube l3 must deliver the differencefrequency, as distinguished from the sum of the frequencies in! and Hi,to its output circuit 3|. This insures that the heterodyne conversionremoves the frequency modulation com- I ponent of the wave from l6.Converter tube 14 maydeliver to transformer I5 the sum or differencefrequency between the frequencies fed fro transformer 2 and oscillatorl5.

Although a transformer with a band-pass characteristic is shown for theretard circuit IS in Fig. 1, any type of band-pass or low-pass filternetwork may be used. Likewise a simple tuned circuit may be used at IS.The main requirement is that the output phase vary with the fre quencyof the input.

Fig. 2b shows a. circuit which may be substituted in place of the tunedtransformer l6 of Fig. 1. This circuit comprises a plate supply choke59, and in parallel therewith a condenser 60 and inductance 62 which areseries'tuned to the carrier frequency. The choke 59 is connected at itsupper end to anode ll of diode M. The lower end of choke 59 returns tothe grounded end of the diode cathode resistor 59'. This type of circuitproduces an amplitude-frequency characteristic as shown in Fig. since inthe presence of carrier frequency output from tube M the series tunedcircuit-is of low impedance and its impedance increases as the carrierfrequency deviates. The phase characteristic of this type of circuit issuch that the polarity is reversed on either side of the carrierfrequency; in other words, as the carrier goes through the frequency Fc,there is a degree phase shift. With this type of characteristic, thecombination with the unmodulated carrier in the phase modulationdetector is such that the output of converter l3 subtracts from theunmodulated carrier from 15 on one side of the carrier frequency, Fe,and adds to the unmodulated carrier on the other This causes amplitudemodulations with envelopes 180 degrees out of phase to be fed to the twodiodes 38 and 39. Further description of I this type, of phase detectionis contained in my U. S. Patent 2,060,611.

Another way of looking at the operation of the circuit with the networkof Fig. 2b replacing l6 is'to -consider the output of converter 13 asconsisting of carrier-eliminated amplitude modulation. With the circuit16 of Fig. 1 replaced by .the .circuit l6 of Fig. 2b it can be seen thatthe output of converter l3. is carrier-eliminated amplitude modulationwhen it is realized that the output of'the network is zero at carrierfrequency and only allows an output to be produced when the frequency ismodulated. The frequency variations are then removed in the heterodyningprocess effected by converter tube l3 so that the resulting output frommixer tube I3 is amplitude modulation sidebands with carrier removed.When these sidebands are re-combined in the circuits 34 with the carrierfrom oscillator [5, the amplitude modulation is ready for detection.With this type of reception, the phase relation between the sidebandsfrom mixer stage I3 and the carrier from oscillator I5 is adjusted sothat amplitude modulations will be produced at the inputs of diodes 38and 39, which have their envelopes 180 degrees out of phase. This phaserelation may be obtainedby means of reactances 50 and 5|, or 53 and 36.

What is claimed is:

1. In an angle modulated wave receiver, a wave frequency converterhaving an input and anoutput, a s0urce of oscillations coupled to saidinput; connections for impressing angle modulated wave energy on saidinput, said energy having predominantly the characteristics of frequencymodulation, a circuit which converts said angle modulations intocorresponding phase deviations coupled to said output, a second wavefrequency converter excited by said angle modulated wave energy and bywave energy passed by said circuit ccupledto the output of said firstconverter, and a phase modulated wave demodulator coupled to said secondwave frequency converter.

2. In receiver apparatus for converting wave length modulated waveenergy having predominantly the characteristics of frequency modulationinto wave length modulated wave energy hav} ing predominantly thecharacteristics of phase modulation and detecting said phasemodulations, circuits for heterodyning said first mentioned wave energywith wave energy of substantially constant frequency, connections forpassing the energy resulting from said heterodyning process through acircuit the electrical length of which varies with variations infrequency of the passed wave energy thereby to superimpose thereonmodulation'having the characteristics of phase modulated wave energy, afrequency converter for mixing said last named wave energy with saidfirst mentioned wave energy, means for deriving wave energy havingpredominantly the characteristics of phase modulated energy from saidlast mixing process, and a phase modulation demodulator responsive tosaid last named wave energy.

3. In a frequency modulated wave demodulator, a source of wave energymodulated in frequency in accordance with signals, a source of waveenergy substantially constant frequency, a rectifier coupled to-both ofsaid sources for heterodyning wave energy from both of said sources toderive wave energy of a difference frequency, a circuit the electricallength of which varies with variations in frequency of the energy passedthereby, coupled to said rectifier, a second rectifier coupled to saidlast named circuit and to said first named source for heterodyning thewave energy passed by said circuit with frequency modulated wave energyfrom said first source to derive the difference frequency, and a phasemodulated wave demodulator coupled to said second rectifier.

4. In a frequency modulated carrier wave receiver, a frequency converterhaving an input circuit and an output circuit, a source of localoscillations of high frequency coupled to said input circuit, a sourceof frequency modulated wave energy, connections for impressing saidmodulated wave energy on said input circuit, a retard circuit whichconverts frequency modulations into corresponding phase deviationscoupled to said energy having predominantly the characteristics of phasemodulation, a source of said first named modulated wave energy, meansfor heterodyning said first modulated wave energy with wave energy ofsubstantially constant frequency, connections for passing the modulatedwave energy resulting from said heterodyning process through a retardcircuit the electrical length of which varies with variations infrequency of the passed wave energy thereby to superimpose thereonmodulation having the characteristics of phase modulated wave energy, afrequency converter for mixing said last named phase modulated waveenergy with said first modulated wave energ means for deriving modulatedwave energy having predominantly the characteristics of phase modulatedenergy from said conversion process, and a phase modulation demodulatorresponsive to said last named phase modulated wave energy.

MURRAY G. CROSBY.

